1. Field of the Invention
The present invention relates to a switching circuit for use in a power converter such as an inverter, a converter, a chopper and the like. The present invention also relates to a power converter using one of them, and more specifically, relates to the switching circuit and the power converter suitable for use with a switching device having normally-on characteristics.
2. Related Art
A wide band-gap semiconductor device using a wide band-gap material such as silicon carbide and gallium nitride or diamond, e.g., a junction field effect transistor (JFET), a static induction type transistor (SIT), a metal-semiconductor field effect transistor (MESFET), a hetero-junction field effect transistor (HFET) and a high electron mobility transistor (HEMT), has excellent characteristics as a switching device. However, the switching device composed of such wide-band-gap semiconductor device has normally-on characteristics. That is, a predetermined negative voltage has to be applied to a conduction control terminal (gate or base) of the switching device composed of the wide-band-gap semiconductor to turn off the conduction control terminal.
So, in the switching circuit and the power converter using the aforementioned switching device, a predetermined negative voltage must be applied to turn off the switching device. One conceivable solution is to fabricate a circuit structure provided with a power source for the conduction control terminal, beside a main power source, to apply the negative voltage to the conduction control terminal. However, such a circuit structure inevitably encounter increase in size and cost because the power source for the conduction control terminal needs an additional area for its implementation. Desirably, a circuit structure that enables to reduce the size and cost may be provided by a built-in power source for supplying a negative voltage to the conduction control terminal.
As for built-in structure of the power source for the conduction control terminal of the switching device having the normally-on characteristics, there is known a method of forming the power source for the conduction control terminal by a capacitor and of charging the capacitor from the main power source as described in Japanese Patent Laid-open No. 2007-288992 for example. Japanese Patent Laid-open No. H8-51770 discloses another known example of built-in power supply structure for the conduction control terminal of the switching device.
In this method, a capacitor can be used effectively as the power source for the control terminal to reduce the size and cost of the switching circuit and power converter since a built-in structure is provided for supplying a negative voltage to the conduction control terminal of the switching device. However, applicable circuit structures are limited in the conventional built-in power source structure disclosed in Japanese Patent Laid-open No. 2007-288992.
That is, in the built-in power source structure disclosed in Japanese Patent Laid-open No. 2007-288992, electricity is charged to a power-supply capacitor, which is to supply a negative voltage to a gate of a lower arm switching device, from a power-supply capacitor, which is to supply a negative voltage to a gate of an upper arm switching device. Although this structure is applicable to a power converter having the circuit structure having the upper and lower aims such as an inverter and a converter, it is not applicable to a power converter such as a boost chopper (this structure will be referred to as a ‘single-arm structure’ tentatively in the present specification). So, this structure is not desirable for use in wide variety of circuit structures.